Estimation of the fossil-fuel component in atmospheric CO<sub>2</sub> based on radiocarbon measurements at the Beromünster tall tower, Switzerland

Abstract. Fossil fuel CO2 (CO2ff) is the major contributor of anthropogenic CO2 in the atmosphere, and accurate quantification is essential to better understand the carbon cycle. Since October 2012, we have been continuously measuring the mixing ratios of CO, CO2, CH4 and H2O at five different heights at the Beromünster tall tower, Switzerland. Air samples for radiocarbon (Δ14CO2) analysis have also been collected from the 212.5 m sampling inlet of the tower on a bi-weekly basis. A correction was applied for 14CO2 emissions from nearby nuclear power plants (NPPs), which have been simulated with the Lagrangian transport model FLEXPART-COSMO. The 14CO2 emissions from NPPs offset the depletion in 14C by fossil-fuel emissions resulting in an underestimation of the fossil-fuel component in atmospheric CO2 by about 16 %. An average observed ratio (RCO) of 13.4 ± 1.3 mmol/mol was calculated from the enhancements in CO mixing ratios relative to the clean air reference site Jungfraujoch (ΔCO) and the radiocarbon-based fossil-fuel CO2 mole fractions. This ratio is significantly higher than both the mean anthropogenic CO/CO2 emission ratios estimated for Switzerland from the national inventory (7.8 mmol/mol for 2013), and the ratio between in-situ measured CO and CO2 enhancements at Beromünster over the Jungfraujoch background (8.3 mmol/mol). Differences could not yet be assigned to specific processes and shortcomings of these two methods but may originate from locally variable emission ratios as well as from non-fossil emissions and biospheric contributions. By combining the ratio derived using the radiocarbon measurements and the in-situ measured CO mixing ratios, a high-resolution time series of CO2ff was calculated exhibiting a clear seasonality driven by seasonal variability in emissions and vertical mixing. By subtracting the fossil-fuel component and the large-scale background, we have determined the regional biospheric CO2 component that is characterized by seasonal variations ranging between −15 to +30 ppm. A pronounced diurnal variation was observed during summer modulated by biospheric exchange and vertical mixing while no consistent pattern was found during winter.